1 © 2006 brooks/cole - thomson chemistry and chemical reactivity 6th edition john c. kotz paul m....
TRANSCRIPT
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© 2006 Brooks/Cole - Thomson
Chemistry and Chemical Reactivity 6th Edition
John C. Kotz Paul M. Treichel
Gabriela C. Weaver
CHAPTER 23
Nuclear Chemistry
© 2006 Brooks/Cole Thomson
Lectures written by John Kotz
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Nuclear Chemistry
Pictures of human heart before and after
stress using gamma rays from radioactive
Tc-99m
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Why do you care?• PET scans• Nuclear Power• Space travel• Smoke Detectors (Am-241)• Ionizing Radiation and X-rays• Neutron Activation• Exposure (pilots, nuclear accidents, Radon)• Carbon Dating• Nuclear Weapons
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Nuclear Radiation
• The Process of emitting energy in the form of waves or particles.
• Comes from the Nucleus of the Atom– The Neutrons– Instability – Binding Energy– E=mc2
– Non-conservation of Mass
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ATOMIC COMPOSITION• Protons– positive electrical charge– mass = 1.672623 x 10-24 g– relative mass = 1.007 atomic mass units
(amu)• Electrons
– negative electrical charge– relative mass = 0.0005 amu
• Neutrons– no electrical charge– mass = 1.675523 x 10-24 g– relative mass = 1.009 amu
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Isotopes
• Atoms of the same element (same Z) but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n: 105B
• Boron-11 (11B) has 5 p and 6 n: 115B
10B
11B
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Radioactivity
• One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of
Marie Curie (1876-1934). • She discovered
radioactivity, the spontaneous disintegration of some elements into smaller pieces.
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Types of Radiation
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Penetrating Ability
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Nuclear Reactions• Alpha emission
Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.
Nucleons are rearranged but conserved
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Nuclear Reactions• Beta emission
Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.
How does this happen?
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Other Types of Nuclear Reactions
Positron (0+1b): a positive electron
K-capture: the capture of an electron from the first or K shell
An electron and proton combine to form a neutron.0
-1e + 11p --> 1
0n
207 207
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Radioactive Decay Series
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Stability of Nuclei• Heaviest naturally
occurring non-
radioactive isotope is
209Bi with 83 protons
and 126 neutrons
• There are 83 x 126 =
10,458 possible
isotopes. Why so few
actually exist?
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Stability of Nuclei
• Up to Z = 20 (Ca), n = p (except for 73Li, 11
5B, 199F)
• Beyond Ca, n > p (A > 2 Z)
• Above Bi all isotopes are radioactive. Fission leads to
smaller particles, the heavier the nucleus the greater the
rate.
• Above Ca: elements of EVEN Z have more isotopes
and most stable isotope has EVEN N.
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Stability of Nuclei
• Suggests some PAIRING of NUCLEONS• Something inside the nucleus gives each atom a
probability of radioactive decay
Even Odd
Odd
Even
Z N
157 52
50 5
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Band of Stability and Radioactive Decay 243
95Am --> 42a + 239
93Np
a emission reduces Z
b emission increases Z
6027Co --> 0
-1b + 6028Ni
Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture
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Binding Energy, Eb
Eb is the energy required to separate the
nucleus of an atom into protons and
neutrons.
Use E=mc2
Find the mass of the isotope.
Sum the masses of the nucleons.
For m, use the DIFFERENCE between those
masses.
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Calculate Binding Energy
For deuterium, 21H: 2
1H ---> 11p + 1
0n
Mass of 21H = 2.01410 g/mol
Mass of proton = 1.007825 g/mol
Mass of neutron = 1.008665 g/mol
∆m = 0.00239 g/mol = 2.39x10-6 kg/mol
c = 3x108 m/sec
From Einstein’s equation:
Eb = (∆m)c2 = 2.15 x 1011 J/mol
How much binding energy is there per nuclear particle?
Eb per nucleon = Eb/2 nucleons
= 1.08 x 108 kJ/mol nucleons
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Half-Life
• HALF-LIFE is the time it takes for 1/2 a sample to disappear.
• The rate of a nuclear transformation depends only on the “reactant” concentration. It does not depend on any factors outside the nucleus.
• Half-life is a property that can be used to identify an element.
• Half-life cannot predict the likelihood a single atom will decay
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Half-Life
Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?
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Kinetics of Radioactive Decay
Activity (A) = Disintegrations/time
N is the number of atoms
Decay is first order, and so
ln (A/Ao) = -kt or
ln (A) – ln (Ao) = -kt
The half-life of
radioactive decay is
t1/2 = 0.693/k
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Radiocarbon DatingRadioactive C-14 is formed in the upper atmosphere
by nuclear reactions initiated by neutrons in cosmic radiation
14N + 1on ---> 14C + 1H
The C-14 is oxidized to CO2, which circulates through the biosphere. There is a constant % of C-14 in the atmosphere. While a plant is alive, it has the same % of C-14 in it as the atmosphere.
When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with t1/2 = 5730 years.
Activity of a sample can be used to date the sample.
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Radiocarbon Dating
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Man-made Eyes to See Small Things
• Humans needed to find a way to extend their senses, to gather knowledge about things beyond our physical constraints.
• Light can be thought of as a piece of information sent between matter.
• The wavelength/frequency/energy of light determines how it interacts with matter and also predicts where it came from.
• Certain materials can “see” light that our eyes cannot.
• Using these materials we learn about the elements in space and on earth.
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Human Limitations
• The molecules in our eyes only work within a very specific range of wavelengths.
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Our Sun- Seen by Ultraviolet Light
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Extending Our Vision
• Common detector materials that interact with light:
• Sodium Iodide crystal:
• Plastic scintillator:
• Germanium Crystal:
• Silicon:
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Cosmic Rays
• Super fast particles from the sun and outer space (protons and ions)---
• Strike the atmosphere and become pions (positively charged fundamental particle), then muons (heavy electrons).
• Built a detector to “see” them using a plastic scintillator.
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• Obtainable info:– Direction of radiation– Shielding effects
• Pyramids example• Depth inside Earth
– Solar activity levels200 muons/m2/second
Protonfrom sun
Pion
Molecule inatmosphere
Muon
Neutrino
Atom ofHydrocarbon
Light
Cosmic Rays
Photomultiplier Tube (PMT)
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Cosmic rays are the source of C-14 used in radiocarbon dating!
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Terrestrial Radiation
• Obtainable info:– Naturally occurring radioactive isotopes can be identified.– Composition of isotopes in rocks is compared to rocks
from around the world. – Background radiation in the air can be measured – Investigation of radiation in the ground.
• Uses gamma ray spectroscopy to “see” light that comes from matter in the ground
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Summary• Certain materials interact with the light that our eyes
don’t detect.• Devices made from these materials have lead to the
field of spectroscopy, meaning “seeing light.”• All modern devices convert a light signal into an
electrical signal.• The electrical signal is arranged in a way that allows
us to ‘see’ what is going on with our eyes.
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Bubble ChambersAlpha, Beta, and Gamma Particles rip through a supercooled gas, ionizing them, and forming bubbles.
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Artificial Nuclear Reactions
New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B.
Radioisotopes used in medicine are often made by these n,g reactions.
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• Applications:– Test for the presence of
heavily shielded dangerous nuclear material.
– Create small amounts of elements (alchemy)
– Find approximate percent compositions of elements in a substance.
Neutron Activation– Shoot neutrons into a substance, stuffing them into a
nucleus to make it unstable. They will then decay in a special way that we can “see” what is in them.
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Transuranium Elements
Elements beyond 92 (transuranium) made
starting with an n,g reaction
23892U + 1
0n ---> 23992U + g
23992U ---> 239
93Np + 0-1b
23993Np ---> 239
94Np + 0-1b
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Transuranium Elements
& Glenn Seaborg
106Sg
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Nuclear Fission
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Nuclear Fission
Fission chain has three general steps:
1. Initiation. Reaction of a single atom
starts the chain (e.g., 235U + neutron)
2. Propagation. 236U fission releases
neutrons that initiate other fissions
3. Termination.
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Nuclear Fission &
Lise Meitner
109Mt
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Nuclear Fission & POWER
• Currently about 104 nuclear
power plants in the U.S. and
about 400 worldwide.
• 17% of the world’s energy
comes from nuclear fission.
• What are would be the benefits
and drawbacks to using nuclear
FUSION instead of nuclear
fission?
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Nuclear Medicine: Imaging
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BNCTBoron Neutron Capture
Therapy• 10B isotope (not 11B) has the ability to
capture slow neutrons
• In BNCT, tumor cells preferentially
take up a boron compound, and
subsequent irradiation by slow
neutrons kills the cells via the energetic 10B --> 7Li neutron capture reaction
(that produces a photon and an alpha
particle)
• 10B + 1n ---> 7Li + 4He + photon
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Food Irradiation
•Food can be irradiated with g rays from 60Co or 137Cs.
• Irradiated milk has a shelf life of 3 mo. without refrigeration.
•USDA has approved irradiation of meats and eggs.
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Effects of Radiation
Rem: Quantifies biological tissue damageUsually use “millirem”
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